Luo He-Lie

Preparation and properties of pure nanocomposite Fe-SiO2 using the sol-gel method

Abstract Pure nanocomposite Fe-SiO 2 has been prepared by the sol-gel method, which is capable of producing large quantities of nanocomposite material. The average diameter of iron particles ranged from several nanometers to tens of nanometers, which can be controlled by the iron concentration and calcining temperature. The results of X-ray diffraction and Mossbauer spectra showed that the nonferromagnetic phase Fe 2 SiO 4 is formed when the iron concentration is beyond about 16 wt% for the samples reduced at 700°C for 4 h. The dependence of the magnetization of iron in Fe-SiO 2 on iron concentration between 1 and 33 wt% was studied for samples reduced at 700°C for 4 h. The dependence of saturation magnetization of iron on temperature between 1.5 and 300 K was measured using an extracting-sample magnetometer for a 2.5 wt% Fe Fe-SiO 2 sample reduced at 700°C for 4 h. A very sharp increase in magnetization was observed when the temperature was below about 40 K. The magnetization dependence on temperature has been well fitted by Blochs law when the temperature is above 150 K, and the Bloch constant fitted from experimental data is 2.33 × 10 -5 K 2 3 , far larger than the value of 3.3 × 10 -6 K 2 3 for bulk iron.

Electronic structures and magnetic properties of Fe2P and the effects of doping with Ni

The first-principles self-consistent cluster method has been used to study the electronic structures and magnetic properties of Fe2P and the effect of doping with Ni. We find that, for Fe2P, the local magnetic moments of FeI atoms at the tetrahedral sites are smaller than those of FeII atoms at the pyramidal sites owing to the greater hybridization of FeI with P; when Ni atoms replace FeI atoms, the FeII magnetic moment shows a slight increase and the ferromagnetism of the (Fe1-xNix)2P compounds is strengthened. However, the substitutions of Ni in FeII sites reduce the FeI atom magnetic moment substantially. These results are in good agreement with experiments.

Electronic structure and magnetic properties of MnBi(Al,Nd)

The electronic structure and magnetic properties of MnBiAl and MnBiNd were studied using molecular cluster model within the local density functional approximation. The obtained results of MnBi are in good agreement with ASW and experiments. Influences of Al and Nd impurities were discussed. The Bi 6p band was found much polarized and the magnetic moment decreased by Al and Nd atom which is attributed to the electron transfer between majority band and minority band.

The electronic structure and magnetic behaviour in the Mn-containing cubic Laves-phase pseudobinary compound of YFe2

The fully self-consistent discrete variational method within the local-spin-density framework has been employed to obtain the electronic structure and magnetic moments of the Mn-containing cubic Laves-phase pseudobinary compound of YFe2. The embedded-cluster model was used to simulate the local environments around Fe in this compound. The calculated results show that the substitution of Mn atoms on Fe sites weakens the ferromagnetism of this compound. With the change of the local environments around Fe. We find that when Fe has one or two Mn atoms in its nearest-neighbour sites, its magnetic moment decreases rapidly, while the Mn moment is 071 mu B, parallel to the Fe moment; when Fe has four or more than four Mn atoms in its nearest-neighbour sites, the moments of these Mn atoms are reversed and antiparallel to the Fe moments and meanwhile the Fe moments increase. In addition, calculations for the cluster with an Mn atom occupying a Y site indicates that such an Mn atom is in a high-spin state. Based upon our results, the various experimental data have been satisfactorily explained.

Cluster studies on the electronic and magnetic properties of MnBi and MnBiAl

Abstract The electronic structure and magnetic properties of MnBi and MnBiAl were studied using the molecular cluster model within the local density functional theory. The influence of cluster size on the calculation results is also discussed. We got quite satisfactory results compared with ASW and experiments. Influences of Al impurity were also studied using different Al content and locations. The main effect of doped Al atoms is the polarization of the Bi 6p band and decrease of the magnetic moment of Mn which is attributed to the charge transfer from the majority band to the minority band. The possible reason for the enhancement of the Kerr rotation in MnBiAl is discussed.

The effect on Si on the local magnetic moment and hyperfine interaction of BCC-Fe

The electronic structure and local magnetic properties of disordered Fe-rich Fe-Si alloys were calculated using the first-principles self-consistent cluster method. The calculated local magnetic moment, isomer shift and hyperfine field are compared with the existing experimental results. The Fe 3d magnetic moment was found to decrease with the number of Si atoms appearing in the first and second neighbouring shells. The isomer shift increases and the hyperfine field decreases in magnitude with an increasing number of Si atoms in the cluster.

First-principles calculations of the local magnetic properties of face-centred cubic disordered Fe-Pd alloys

Self-consistent field electronic structure calculations were performed for embedded clusters representing FCC disordered Fe-Pd alloys. The discrete variational method was employed in the framework of local-spin-density theory. The hyperfine magnetic fields and the magnetic moments on several distinct Fe sites and Pd sites in the Fe-Pd alloys were derived from the calculations. The results for magnetic moments show that the Fe-Pd alloys are strong ferromagnets and that the Fe magnetic moment is almost independent of the number of Pd nearest neighbours, slightly increasing with increasing number of Pd nearest neighbours, while Pd atoms are strongly polarized. When a Pd atom is surrounded by six Fe and six Pd nearest neighbours, it has a maximum induced magnetic moment. The changes in hyperfine fields due to the variation in the number of Pd nearest neighbours are in good agreement with experiment.

Self-consistent cluster calculations of local magnetic properties of non-equilibrium crystalline FCC Cu-Fe and FCC Ag-Fe alloys

The first-principles spin-polarized discrete variational Xalpha method was used to calculate the electronic structures at a central Fe site in a number of embedded clusters representing FCC Cu-Fe and FCC Ag-Fe alloys. The magnetic moment and isomer shift were derived from the calculations and compared with the existing experimental data. The calculated results show that the cluster model yields a magnetic moment and isomer shift consistent with experimental magnetization and Mossbauer data. The Fe 3d magnetic moment and isomer shift in Cu-Fe alloys decrease slightly on increase in the Cu concentration. The Fe 3d magnetic moment in Ag-Fe alloys is larger than that in Cu-Fe alloys. There is an enhancement of the magnetic moment in Ag-Fe alloys.

On the magnetic anisotropy of the Co and γ-Fe2O3 ultrafine particles

The anisotropy constants K of γ-Fe2O3, Co-γ-Fe2O3 and Co ultrafine particles precipitated in Cu-Co alloy have been measured by using the equations for describing the relaxation of superparamagnetic particles. It is found that the anisotropy constant K of Co-γ-Fe2O3 fine particles is greater than that of γ-Fe2O3 by about 50%; the K value of Co ultrafine particles of a radius equal to 36 A is greater than that of to 48 A by about 100%. This phenomenon has been interpreted as the existence of surface anisotropy of the Co ultrafine particles.

Relaxation process in cobalt-ferrite epitaxial iron oxides

The migration process of Co ions in the cobalt-ferrite epitaxial layer on γ-Fe2 O3 powder has been studied by magnetic thermal annealing method. The activation energy E and coefficient A of Co ions have been measured. An abnormal increase of coercivity of sample annealed in a transverse magnetic field was found. On the basis of Mossbauer and XPS results the reason of this phenomenon is discussed.